Proper differentiation of gonadal tissues is a prerequisite for normal testis and ovarian development, which is in turned required for successful sexual reproduction. In mammals, the complement of sex chromosomes (XX or XY) in the bipotential fetal gonadal determines sex and dictates whether male or female- fate promoting genes are expressed. The consequent sexual differentiation process is initiated early in development and concludes in puberty. When differentiation fails, the resulting clinical disorders can be debilitating. Congenital conditions stemming from irregular sexual differentiation (also known as disorders of sexual development, or DSD) involve complete or partial sex-reversal leading to gonadal dysgenesis and a potentially increased risk of specific types of cancers (Looijenga et al., 2007). Several sex-regulatory genes have been discovered (Tannour-Louet et al., 2010), but the genetic basis of most DSD cases remains unknown. One likely player is the conserved transcription factor Dmrt1. Deletions of the human chromosome 9p region containing DMRT1 cause testicular dysgenesis and sex reversal phenotypes (McDonald et al., 1997; Raymond et al., 1999), and male mice lacking Dmrt1 develop similarly abnormal gonadal tissues (Raymond et al., 2000). Only recently has it begun to be appreciated that sex differentiation is not a unidirectional process. In fact, somatic cell sexual fates are actively maintained in the adult gonads. The Zarkower/Bardwell group recently discovered that Dmrt1 is essential for this maintenance program in the mouse testis. Loss of Dmrt1 results in male-to-female cell fate reprogramming, with testicular cells transdifferentiating to form their ovarian counterparts (Matson et al., 2011). Moreover, preliminary studies indicate that the presence of ectopic DMRT1 in the adult ovary is sufficient to cause extensive female-to-male transdifferentiation. Together these results suggest that Dmrt1 anchors the male sex maintenance system. The goal of this research training proposal is to define, in detail, the genetic networks controlled by Dmrt1 which regulate sexual transdifferentiation and sex maintenance in the gonad. To achieve this goal, technologies including conditional gene knockout and gene activation, expression profiling by RNA-seq, and RNAi by in vivo viral transduction in the testis will be employed. Use of a combination of precise genetic manipulations and powerful genomic technologies will facilitate the discovery of gene functions and help define the regulatory mechanisms essential for control of sexual cell fate. The Aims of this proposal are to: 1) identify gene networks that control transdifferentiation and male sex maintenance in the postnatal gonad, and 2) determine how ectopic DMRT1 expression masculinizes female gonadal tissues. The findings from this work should have direct relevance to other examples of cell fate reprogramming and to the diagnosis and treatment of human DSD, infertility, and gonadal cancers.